PoS(NIC XI)008 http://pos.sissa.it/ galaxies and the globular cluster abundance ratios. I then discuss ecause there is some disagreement follows the thick disk trend. cal evolution, necessary for under- a few interesting subtopics. I have these dwarf systems. onged chemical evolution in a leaky discussion of the vast field of obser- e Commons Attribution-NonCommercial-ShareAlike Licence. Carnegie Observatories ∗ [email protected] Speaker. over the measured abundance ratios, except for [O/Fe] which vational chemical evolution, so Icompletely have omitted limited the this Galactic paper bulge; to this may be as well, b In this review I first outline some interesting ideas in chemi Unfortunately, space limitations prevent a comprehensive standing the evolution ofabundance galaxies results from from measured studies of elemental Omega Cen. stars in Finally, I Local present Groupbox a that dwarf qualitative can scenario explain the of observed prol abundance ratios trends in ∗ Copyright owned by the author(s) under the terms of the Creativ c

Andrew McWilliam Galactic chemical evolution: the observational side 813 Santa Barbara Street Pasadena, CA 91101, USA E-mail: 11th Symposium on Nuclei in the19-23 Cosmos July 2010 Heidelberg, Germany. PoS(NIC XI)008 - LMC and nd they are Bergh (1962) tellar masses, or tion; however, in completion; thus, 1.6 dex, which he as, and a constant le 1955; Burbidge Andrew McWilliam imple Model; this f the Galactic disk, ity of the dwarfs is − by Nick Prantzos at stars were produced, ss of metals from its 1972) introduced the produced by stars and neously, and homoge- 3. The observed solar e. There is no inflow or ing by many generations and a good starting point − rstand some basic qualitative 2 . The widely accepted explanation for the G-dwarf 5 Gyr (e.g., Sommer-Larsen 1991). At early times there ∼ 0.6 dex, respectively (e.g., Russell & Dopita 1990). The presence of − “G-dwarf problem” . Thus, the average of the metallicity distribution function (MDF) for a stellar 0.3 and , which is the ratio of the mass of metals produced to the mass locked-up in low − “yield” terminated halo evolution at low metallicity. “yield” Chemical evolution models with increasing complexity have been constructed, a This paper is mostly concerned with the observational side of chemical evolu Perhaps the most basic chemical evolution model is the Simple Model of van den While the mean metallicity of a Simple Model canThe be mean used [Fe/H] to for the determine Galactic the halo, yield, found in by Hartwick stellar (1976), is near In the 1950s both observations and theory (Chamberlain & Aller 1951; Hoy The Simple Model age-metallicity relation is linear, while the MDF is logarithmic: for ex This contrasts with the explanation for the metallicities of gas and young stars in the was relatively little gas present so only small numbers of metal-poor G-dwarf compared to later epochs when the metallicity was higher. Galactic chemical evolution: the observational side 1. An Introduction to Chemical Evolution problem is that inflow of low-metallicity gaswith has an occurred exponential during the decay evolution timescale o of et al. 1957; Preston 1959) indicatedthat that the most Galaxy of the had chemical undergone elements chemicalof were evolution stars. due to nuclear process 1.1 The Simple Model now much more realistic; forthis discussion conference. of However, these I I find refer thefor Simple the thinking Model reader about a to ideas useful the in heuristic talk chemical tool evolution. 1.2 The Metallicity Distribution Function systems the mean of the MDF can be affected in several ways: order to interpret the measured abundance ratiosconcepts it is in necessary chemical to evolution unde theory. neighborhood MDF shows a deficiencydeficit in is metal-poor known stars as compared the to the S ample, there are a thousand stars at [Fe/H]=0 for each star at [Fe/H]= and Schmidt (1963.) This idealizedrate scenario of begins star with formation a mass (SFR),initial of in mass metal-free stellar function g (IMF). generations, Each withmassive stellar a stars, generation fixed produces and distribution an fossil of identical remnantsneously, s ma from mixed into the the dwarfs; unused gas, the thusoutflow metals increasing of are the material gas instanta to metal the abundanc region,concept i.e., of it is a “closed box”. Searle & Sargent ( equal to the system indicates the yield of metals from its stars. mass stars. In a Simple Model that runs to gas exhaustion, the average metallic explained as due to the halogas losing outflow its gas before chemical evolution could go to SMC, at [Fe/H]= PoS(NIC XI)008 esis. , while the 1, followed ive stars to 1, indicates mass of iron ⊙ B90’s gentle should show a e most massive ∼− (Si, S, Ca, and ∼− have had time to 35M from O to Ti, are pted in Figure 1, Andrew McWilliam e SNIa progenitors ∼ e experienced huge the SNIa time-delay, m the time delay be- nhancements of Mg, igh mean metallicity, e Figure 1). nsistent with detailed [Fe/H] f the LMC and SMC; produce larger [O/Fe] s with increasing SNII while the [Fe/H] of the s prevailed; subsequent n [Fe/H]. rs, both of which varies is idea Wyse & Gilmore s, where O is produced, lculations (e.g., Woosley (O and Mg), made in the explosive ould depend on the SFR. o O. A Simple Model lin- fractions suggest prolonged nd hydrostatic 3 0. The trend is, actually, a composite of the ratios ∼ 0 at [Fe/H] ∼ /Fe] α -elements, although no single nuclear reaction is responsible for their synth α 0.1 Gyr, a significant effect on [O/Fe] does not occur until much later. ∼ /Fe] versus [Fe/H] in the solar neighborhood show a plateau below [Fe/H] α MB90’s strict age-metallicity relation (probably unrealistic), meant that only th The alpha elements can be divided into two categories: The calculations of Matteucci & Brocato (1990, henceforth MB90), ada Tinsley (1979) proposed that the trend of [O/Fe] with [Fe/H] resulted fro Wallerstein (1962) and Conti (1967) first recognized the factor of two e Changes in the IMF can also affect the mean metallicity: the yield depends on the 1 Gyr delay for the onset of significant SNIa Fe production, roughly co ∼ Ti), principally made during the explosion.& Supernova Weaver nucleosynthesis 1995) ca show that O and Mg are made by progenitors with masses envelopes of massive SNII progenitors before the explosive event, a stars contributed to the composition atslope to the higher lowest [O/Fe] metallicity. below This themass. is knee, the because the Massive cause O/Fe for SNII yield M produce ratioand higher increase because of [O/Fe] greater due fallback to of(1993) Fe, their showed than larger that lower envelope mass shallower SNII. IMFratios. Following slopes th Thus, (i.e., the [O/Fe] weighted ratios to belowknee massive the can stars) knee constrain can the constrain SFR the (and IMF formation slope, timescale) for a stellar system (se showed that the down-turn, orHigh knee, SFR in systems, like the bulges [O/Fe] andand versus giant [Fe/H] ellipticals, the plot reach decline high sh [Fe/H] indecline before [O/Fe]; in [O/Fe] similarly, at low low [Fe/H]. SFR systems, such as dwarf galaxies, calculations of Matteucci & Greggiohave (1986). delays I note that while the most massiv Si, Ca, Ti and Oreferred in to Galactic as halo RGB stars. These even-numbered elements, a transitioning from halo, to thick disk and thin disk populations. produced by massive stars, andwith the mass the locked-up IMF. in In low-mass thisexplain dwarf how sta way rapid Ballero bulge formation, et with al.near no the Fe (2007) from solar SNIa, employed value. could anmass produce inflows IMF An a early-on, h weighted alternative, giving an to suggested extreme mass by G-dwarf A.Pipino, problem and is higher that mea the2. bulg Alpha Elements Plots of [ by a steady decline to [ Galactic chemical evolution: the observational side young and old stars (supergiants, RGBevolution, and and carbon a stars) low and SFR, high forreach gas these solar dwarf values. galaxies; thus, However, the outflowsrealistic metallicity models could may are also not required explain for the a full low understanding. [Fe/H] o tween SNII and SNIa. At early times,addition and of low SNIa [Fe/H], lowered the the SNII [O/Fe] [O/Fe]ear ratio value age-metallicity because SNIa relation produce and Fe the but observed n [O/Fe] decline, beginning near PoS(NIC XI)008 1 (see Maeder ∼− ] ant amounts ly traced the Andrew McWilliam t supergiants (e.g., -Rayet phenomena. endent stellar winds ll observers, there is ent uncertainty about ld normally be burned sult from a decrease in ted in the Galactic thick cenario suggests a smaller /O] ratio. velope stripping from binary FR, modified from Matteucci & Bro- SNII progenitors. Thus, the ratio of [Mg/Ca] ⊙ 4 Predicted [O/Fe] versus [Fe/H] for systems with different S Early abundance studies of Magellanic Cloud stars were restricted to brigh Thin disk and Galactic bulge [O/Mg] trends show a steep decline above [Fe/H The O yield from massive stars is also predicted to be sensitive to metallicity (e.g., While predicted SNIa yields make effectively no O or Mg, they do make signific 1992), due to the stripping ofIn the this envelope way by the stellar [C/O] yield winds, ratio relatedto increases oxygen to with is the [Fe/H], removed Wolf since from carbon the thatdisk star. wou and This bulge mechanism (Cesctti appears et to have al.mass opera 2009). transfer Wolf-Rayet should stars also produced decrease via the en oxygen yield and increase the [C Spite et al. 1986;very Russell recent & composition; Bessell 1989; typically, McWilliamnon-LTE the effects & on supergiants Williams the were 1989), results. hot, which with on subsequ of Ca (e.g., Nomoto 1984), near theamplitude solar change [Ca/Fe] in ratio. [Ca/Fe] Thus, than Tinsley’s [O/Fe]. s Figure 1: Galactic chemical evolution: the observational side production of Si through Ca peaks near 20–25M 3. The Chemical Composition of Dwarf Galaxies 3.1 The Magellanic Clouds cato (1990). should be sensitive to the IMF slope. McWilliam et al. 2008). While theno disagreement bulge for results the are thin now disk not trend.oxygen agreed The yields upon [O/Mg] decline from by in massive a the stars disk due can(e.g., re to Cescutti envelope et stripping al. by 2009). metallicity-dep PoS(NIC XI)008 re was made. redicted by /Fe] trends for of these dwarf m & Smecker- t the metal-rich as formed from α e predictions of 08) are deficient ne et al. (2007) lay scenario and 0.2 dex deficient, ts, which had not of accreted dwarf ceforth Sgr dSph) Andrew McWilliam ∼ n that [O/Fe] and the aco, Ursa Minor and howed a steep decline all ages; however, initial l. (2010) recently found ern a trend of [O/Fe] with d with Smith et al. (2002). red. Subsequent enrichment . Not withstanding, there are However, the metal-poor Sgr and Ti,/Fe /Fe] trends for the Draco and α /Fe] than the Local Group dwarf galaxies? /Fe] (e.g. Nissen & Schuster 1997; Brown et α α 5 /Fe] ratios, but the older, metal-poor, stars in the α 0.6, but could be described as bimodal, as if halo and − -element enhancements. Similar results for the [O/Fe] ratios 0 for all [Fe/H]. The average [Mg/Fe] LMC trend is deficient, α ∼ 0.2 dex). It was not clear whether the [O/Fe] ratios at the low [Fe/H] end a − /Fe] ratios are deficient in the LMC, relative to the solar neighborhood, as p /Fe], for Mg, Si, Ca and Ti, in the Fornax dSph, although no O measurement α α A large sample of LMC RGB stars studied by Pompéia et al. (2008) provided [ These abundance results are qualitatively consistent with Tinsley’s time-de Bonifacio et al. (2000), Smecker-Hane & McWilliam (2002), and Sbordo The first Local Group dwarf galaxy abundance measurements, for Dr For the more metal-rich stars in the Sagittarius dwarf spheroidal galaxy (hen Later abundance studies of LMC red giant branch (RGB) stars probed Shetrone’s results provoked an important question: if the halo is composed /Fe] deficiencies were found by Smecker-Hane & McWilliam (2002), McWillia α the same in the LMC and Galaxy; this zero-point issue should be resolved. [ The answer is that theyet halo suffered must significant be enrichment made by mostly SNIaplenty of when of early they examples dwarf were of galaxy accreted Galactic fragmen halo stars with low [ low [ Ursa Minor dSphs, and claimedgalaxies. halo-like ratios for the most metal-poor members in the Sculptor dSph were found by Geisler et al. (2005); and Letarte et a nearby dSphs have halo-like compositions. 3.3 Sodium and Aluminium MB90, but further investigation is warranted. 3.2 Alpha Elements in Local Group Dwarf Galaxies al. 1997). Cohen & Huang (2009,2010) found low metallicity knees in the [ results from Hill et al. (1995)[Fe/H]. and Smith Korn et et al. al. (2002), (2002) with 12 werein LMC too [O/Fe] RGB few well stars, to below plus disc that the for earlierend results, the s Milky (near Way, [Fe/H]= with [O/Fe] lower by almost 0.3 dex a Hane (2005a), Sbordone et al.dSph stars (2007), possessed and normal Carretta halo et al. (2010). Galactic chemical evolution: the observational side relative to the Milky WayMB90. (O was not measured), qualitatively consistent with th the predictions of MB90.accreted They dwarfs at are very also early consistent times,of before with present the the bulk day idea of dwarf SNIa that galaxies had the occur decreased halo the w [ Sextans, by Shetrone et al. (2001), gave an average of Mg/Fe, Ca,/Fe thin disk populations overlapped inother [Fe/H]. [ Thus, there is rough confirmatio relative to the Milky Way, below [Fe/H]= O, Mg, Ca, Si and Ti.Relative to The the handful Galaxy of the O/Feat abundance LMC all ratios [Ca/Fe] overlappe [Fe/H]; and remarkably, [Ti/Fe] [Si/Fe] ratios of Pompéia et al. (20 galaxy fragments, then why does it possess higher [ PoS(NIC XI)008 85% SNIa ∼ I progenitors hborhood. For Andrew McWilliam ity) as described by e thin disk, and the ficiencies, relative to 0.3 dex, similar to the but with a metallicity- sley (1979) time-delay gr dSph is k on Al/Fe ratios would − found Na/Fe deficiencies deficient in dwarf galaxies, t, are common-place among -element deficiencies. Large α c bulge, adapted from Fulbright et al. 0.8 dex difference between the bulge and ∼ 0.4 dex, respectively, similar to the deficiencies − 6 0.3–0.4 dex in the Sgr dSph; results of Carretta et al. 0.8 dex. For the Sculptor dSph Geisler et al. (2005) ∼ − 0.5 and − 0.7 dex were found for the stars in the Fornax dSph, by Letarte et al. ∼− [Al/Fe] for the Sgr dSph, Galactic thin disk, and the Galacti For the LMC RGB stars Smith et al. (2002) found a mean [Na/Fe]= In Figure 2 I show [Al/Fe] versus [Fe/H] for three systems: Sgr dSph, th The main source for Al and Na is thought to be the hydrostatic phase of SNI 0.3–0.4 dex for her large sample of LMC RGB stars, relative to the solar neig ∼ results from F–G supergiants by Hillof et al. (1995). Pompéia et al. (2008) the Draco and UMithe dSphs Galaxy, Cohen for the & metal-rich Huang end (2009, of their 2010) sample, found similar Na/Fe to de their [Na/Fe] deficiencies, found mean [Na/Fe] and [Al/Fe] near (2010) are inconclusive. (2007). Figure 2: (2010), for which the mean [Fe/H] was Galactic chemical evolution: the observational side found [Al/Fe] and [Na/Fe] deficiencies of (e.g., Woosley & Weaver 1995), modulated byArnett the neutron (1971). excess (i.e., Thus, metallic Aldependent and yield Na trend imposed; should so, behave it much iswhere not like alpha-element surprising alpha-elements, that deficiencies Al are and also Na observed. are the dwarf galaxies, and that whenbe measured useful. Al is also deficient. More wor found in the Sgr dSph. Thus, it appears that the Na deficiencies, at leas Sgr dSph [Al/Fe] ratios, at thescenario, same if [Fe/H] the values, bulge is [Al/Fe] remarkable. reflects the In pure the SNII Tin ratio, then the Fe in the S Galactic bulge, from Fulbright et al. (2007). The PoS(NIC XI)008 For the dwarf e first noted by 0.5 dex, occurs t), giving a high e neutron-capture ∼ Andrew McWilliam ver, firm detection rocess, with a locus ed large dispersion, , so non-LTE effects ields. In Figure 2 the 0.7 dex. Notably, the ith increasing [Fe/H], ted by SNIa material, s/ls] h SNIa Fe fraction must ] versus [La/H] adapted ∼− in Sgr dSph RGB stars, by am & Smecker-Hane (2005a). Solid sition; dashed line indicates the locus of 95% on. 7 0.3 dex, is due to the s-process, despite the fact the solar + 0. Remarkably, the Ba II lines were too saturated in the Sgr ∼ 1 dex at [Fe/H] + [La/Eu] versus [La/H]for the Sgr dSph, adapted from McWilli 95% r-process. An r-process assignment can only be identified from th ∼ Mild enhancements in heavy s-process elements in the LMC supergiants, wer The same authors found normal Y abundances (a light s-process elemen from McWilliam & Smecker-Hane (2005a), indicating thethat dominance of indicates the halo s-p composition plus at least 95% s-process above [Fe/H] dSph for reliable abundance measurement. Figure 3 shows plot of [La/Eu Eu is element abundance ratios, not the ratio to iron. [Eu/Fe] ratio enhancement, at roughly Russell & Bessell (1989),galaxies McWilliam & the Williams small (1991) numbers andwith of Hill possible stars et slight studied al. enhancements byof in Shetrone (1995). s-process the et enhancements mean al. in heavySmecker-Hane s-process nearby (2001) & noted. dwarf McWilliam show (2002), galaxies showing Howe were a steadyup first increase to seen in [La/Fe] [La/Fe]= w line shows locus of pure s-processs-process added plus to 5% an r-process original added compo to the original compositi [La/Y], which measures the heavy/light, or [hs/ls], ratio; the enhanced [h Figure 3: Galactic chemical evolution: the observational side material; however, if the bulge contains any Febe from even SNIa, greater. then the Thus, Sgr dSp theand Sgr this dSph system iron-peak would be elements usefulRGB seem to stars to compare of be the with domina Sgr predicted dSph SNIaare and iron-peak unlikely the y to bulge explain have the similar Al [Fe/H], and and Na temperature abundance differences. 3.4 S-Process Enhancements PoS(NIC XI)008 ph 0.5, and − /Fe] ratios. I cleus captures α B s-processing. e (2005a). e oldest stars in d dwarf galaxy. r [Cu/Fe] shows trend with [Fe/H] Fe/H]= ted resulted from GB stars by Pom- as had leaked-out, Andrew McWilliam r, AGB stars dom- t of [La/Eu] versus of metal-poor stars nd it seems reason- ratio McWilliam & cluster, Omega Cen sumably, leaky-box up to nearly 0.6 dex, om metal-poor AGB Ursa Minor dSph by nced, [ rly, metal-poor, popu- ificant nucleosynthesis 03). It appears that the tly inconsistent with the rly all stars, a locus that n in the Sgr dSph. These e in Sgr dSph stars with n in the most metal-rich stars 1.5; above that the [Cu/Fe] ratio ∼− 8 0.7 dex and the solar value. -enhancements in Omega Cen may provide a critical clue α − 0.6 dex, below [Fe/H] 0.8 dex. In the same month Geisler et al. (2005) found similar − − 1, which was much lower [Fe/H] than the solar metallicity Sgr dSph − 0.6 or below − McWilliam & Smecker-Hane (2005b) found severe [Cu/Fe] deficiencies, The s-process abundance patterns in these dwarf systems indicates sign Busso et al. (1999) showed that high [hs/ls] ratios occur in metal-poor AG S-process enhancements have long been known for the Galactic globular Together with similar Cu and Mn abundances, this chemical similarity with the Sgr dS The solar neighborhood (i.e., halo, thin and thick disk populations) trend fo Similar s-process enhancements and patterns have been found in the LMC R [Cu/Fe] deficiencies in the Sculptor dwarf galaxy, but at lower [Fe/H] tha in the Sgr dSph for [Fe/H] above remains constant thereafter (see Misheninathin et disk al. [Cu/Fe] ratio 2002; is Simmerer constant, et whilefor al. there the is transition 20 an from approximately halo linear to [Cu/Fe] thick disk populations. increases approximately linearly with [Fe/H], toward the solar composition at [ find this difference bothTinsley puzzling time-delay and scenario; fascinating, thus, because the it is apparen for understanding chemical evolution. (e.g., Vanture, Wallerstein & Brown 1994;[La/H] Smith from et al. the data 2000). ofis I Johnson consistent note with that & a the Pilachowski. plo additionOmega (2010) of Cen, pure shows, similar s-process to for Sgr material nea dSph to (Figure the 3 composition here) of from McWilliam th &suggests Smecker-Han a common history,The and main that chemical Omega difference Cen is may that be Omega the Cen core possesses halo-like, of enha an accrete Galactic chemical evolution: the observational side for Sgr dSph stars for [Fe/H] between a sub-solar plateau, near [Cu/Fe]= péia et al. (2008),Cohen in & Huang the (2010); Fornax thus, dSph thisof by appears dwarf to galaxies. Letarte be et a al. general feature see (2010), and in the by metal-poor AGB stars,leaky which box McWilliam chemical & evolution, Smecker-Hane in (2005a)than which the sugges the solar neighborhood. MDF possessed Initially,lations these a proportional galaxies larger to would the fraction have mass formed ofsuch ea gas, that but the by late ejecta times frominated a the large neutron-capture now fraction relatively element of large the abundance population g chemical pattern of evolution old, must of be metal-poo the quite late-time generalable that and gas. the apply amplitude to of Pre many the dwarf effect galaxies, may a be greater3.5 for lower-mass Manganese galaxies. and Copper RGB stars studied. Thus, the Sgrstars. dSph must have been polluted with ejecta fr At low metallicity the ratiomany of neutrons, iron thus seed pushing nuclei theSmecker-Hane to synthesis (2005a) neutrons to concluded is heavy low, that nuclei. so[Fe/H] the From each near AGB the seed s-processing [La/Y] nu took plac PoS(NIC XI)008 solar 0.4 dex, est [Fe/H] − es and giant e mirror image in these systems, m massive stars is Andrew McWilliam Sgr dSph. Included ncy, near uld possess [Cu/Fe] 10) for the UMi dSph (2002). More recently, sted by the abundances metallicity-dependence , while the production of in LMC stars with [Fe/H] g of SNII progenitors, and (2008); similar trends are also seen in massive stars is the principle ky Way stars. 9 weak sr-process 0.4 dex, relative to the Milky Way. Clearly, Cu deficiencies are a common, ∼ 1, but increases roughly linearly with increasing [Fe/H] to [Mn/Fe]=0.0 at − [Cu/Fe] for the LMC (filled circles) taken from Pompéia et al. and s-process elements. By this same argument, one would expect that bulg 1, almost as if the low [Cu/Fe] ratios in the halo has been extended up to the high α − The Cu deficiencies in dwarf galaxies indicates a paucity of high-mass stars In the solar neighborhood [Mn/Fe] shows approximately constant deficie Bisterzo et al. (2004) concluded that the metallicity (Sobeck et al. 2006; Gratton 1989). Because this trend looks like th below [Fe/H]= although the metallicity-dependence could play aconsistent role. with the This prolonged lack star ofof formation material the and fro leaky box scenario sugge source of Cu in thethat Galaxy, occurring SNIa and during AGB convective shell stars C doof burnin not the produce weak significant sr-process explains quantities the ofiron positive from Cu. slope SNIa The of explains the [Cu/Fe] flattening-out with of [Fe/H] the slope in the thin disk. ellipticals, or systems with highenhancements, SFR relative and/or to an the excess solar of neighborhood. high-mass stars, sho perhaps ubiquitous, signature of dwarf galaxies. in the LMC. Carretta et al.without (2010) discussion have confirmed in the their low paper, [Cu/Fe]are the ratios [Cu/Fe] in deficient the ratios by of Cohen & Huang (20 in Sgr, Sculptor, and UMi dSphs. Other symbols represent Mil Figure 4: Galactic chemical evolution: the observational side deficiencies were reminiscent of those foundPompeia in Omega et Cen al. by Cunha (2008), etabove Figure al. 3 here, found large [Cu/Fe] deficiencies PoS(NIC XI)008 79) and r metal- [hs/ls] s- ctic bulge much of the ed in SNIa. In ence for signif- population. uence lifetimes. in disk to bulge ancements of s- timescale, this is w SFRs and pro- Andrew McWilliam rs eject significant illiam et al. (2003) n overwhelmed by re is very little gas ply to SNIa, which ut deficient in high lusters also indicate -dependent in SNII; alitatively explained h could be explained anted. ky-box chemical evo- 95), are similar to the n deficiency would be en seen in Omega Cen h a leaky box chemical the increase in [Mn/Fe] times to produce higher is. nding in their sample of Sgr 85% SNIa. Thus, the dwarf ∼ 0.2 dex, in stark contrast to the ex- ∼ 10 1. If Gratton’s suggestion is correct, the time delay scenario of Tinsley (19 /Fe] trends, the Na and Al deficiencies, and the low [Cu/Fe] ratios at highe α ∼− /Fe] trend with [Fe/H], Gratton (1989) suggested that Mn is over-produc α In a leaky box at early times a significant metal-poor population is formed, but The low [ The low mean metallicities and high [hs/ls] s-process ratios provide strong evid The chemical properties of the dwarf galaxies, outlined above, can be qu These prolonged formation timescales are supported by the significant enh McWilliam et al. (2003) also found that the [Mn/Fe] trend with [Fe/H] in the Gala Measurements of [Mn/Fe] abundance ratios in Sgr dSph RGB stars by McW /Fe, Al/Fe, Na/Fe, Cu/Fe ratios, and few high metallicity AGB stars producing low gas is lost from theto galaxy form during new subsequent generations evolution, of until stars. at late Thus, times there the are very few SNII at late icant mass-loss from the dwarf galaxiesevolution during the their high evolution. [hs/ls] Without ratios suc fromlow the [hs/ls] metal-poor ratios AGB at stars higher would [Fe/H], have and bee the mean metallicities would be higher. in a model of prolonged chemicallution. enrichment with on-going gas-loss, or lea process material, produced by relativelyIn low the mass case of AGB the stars Sgr with dSpha the long long ages formation main and timescale. metallicities seq of its associated globular c pectations if Mn is over-produced byby SNIa. Cunha Similar et Mn al. deficiencies (2010), have be butdSph Carretta stars. et al. The (2010) low did [Mn/Fe] notdeficiencies ratios confirm seen this in for fi the LMC Sgr supergiants, dSph; by more Hill work et on LMC al. [Mn/Fe] (19 ratiosis is similar warr to the solaralso neighborhood; inconsistent assuming with that the the expectationspredicted if bulge for SNIa formed the over-produce bulge. on Mn, Arnett a (1971) since short concluded aMcWilliam that et M the Mn al. yield is (2003) metallicity speculatedalso that synthesize this the metal-dependence iron-peak should elements. alsoby Thus, ap metal-poor low SNIa, [Mn/Fe] which might ratios be in expected Sgr to dSp accompany the metal-poor AGB 4. A Qualitative Model for Dwarf Galaxy Evolution process ratios. However, by late times the old, metal-poor, population AGB sta this scenario the time-delay for theabove onset [Fe/H] of SNIa would be responsible for showed a deficiency relative to the solar neighborhood trend by α Galactic chemical evolution: the observational side of the [ MB90 would predict that [Mn/Fe]SFR systems, is such enhanced as in bulges and low elliptical SFR galaxies. dwarf galaxies, b galaxies are good places to look for the signature of SNIa nucleosynthes licities, all indicate low SNII/SNIa ratioslonged in evolution. dwarf systems, In consistent with particular,suggests for the that lo the comparison Sgr of dSph [Al/Fe] iron-peak from material Sgr near dSph solar to [Fe/H] is th PoS(NIC XI)008 , , , , 249 , ApJ, ratios in , lost from ls would be ve prolonged MNRAS xpected to be , 486 , me from at late 67 ejecta from vari- n. In particular, a , /O]), although the Andrew McWilliam nce pattern of the of Different Mass slope is everywhere AJ , 418 vide interesting tests hesis of Elements from , lts for dwarf galaxies, strain the sites of nu- ny the old, metal-poor, in dwarf galaxies were ld be helpful to have a ssible to probe the IMF 209 , , 507 d models, as a function of 130 ApJ , , ifferent kinetic energies; also, ApJ , Synthesis of the Elements in Stars , 11 The Atmospheres of A-Type Subdwarfs and 95 Leonis Inferences from the Composition of Two Dwarf Blue Galaxies , 121 On the G-dwarf abundance distribution in the solar cylinder 1 The Chemical Evolution of the Galactic Halo The frequency of stars with different metal abundances , A Spectroscopic Study of the RR Lyrae Stars The Rate of Star Formation. II. The Rate of Formation of Stars ApJS , On Nuclear Reactions Occurring in Very Hot STARS.I. the Synt , 547 , 758 , 25 29 , 137 173 , , , 52 ApJ 114 368 Carbon to Nickel RvMP ApJ In this scenario an important question is: where do the iron-peak elements co Of significant interest is the role of the IMF in dwarf galaxy chemical evolutio Detailed chemical evolution models are required to constrain the amount of gas [1] J.W. Chamberlain & L.H. Aller 1951, [7] L. Searle & W.L.W. Sargent 1972, [2] F. Hoyle 1955, [9] F.D.A. Hartwick, 1976, [5] S. van den Bergh 1962, [6] M. Schmidt 1963, [4] G.W. Preston 1959, [8] J. Sommer-Larsen 1991, [3] E.M Burbidge, G.R. Burbidge, W.A. Fowler, & F. Hoyle 1957 Galactic chemical evolution: the observational side amounts of gas andneutron-capture s-process elements of elements, the younger, and more thus metal-rich, population. these dominate the abunda major question for understanding star formation inthe galaxies same, is but whether with the reduced IMF high-masswith end cutoffs various in element dwarf ratios systems. that Itinterpretation is are may po sensitive require to modelling. stellar Comprehensiveand mass CNO other abundance (e.g., element resu [Mg/Ca], ratios, such [C asof [Rb/Zr] the and paradigm D/H outlined in here. these systems would pro References these dwarf galaxies. Other phenomenaous sources, to such be as modelled AGB includethe stars, the SNIa, effects retention and of of SNII, dark which mattervery have very and helpful d cooling for (i.e., understanding metallicity)cleosynthesis the for on evolution various gas of elements, retention. these andset constraining The galaxies of stellar mode and predicted yields. abundance to ratios Itsome con for mass-loss wou parameter. dwarf So galaxies far, from mostobserved, such of not the detaile predicted. abundance anomalies found times? 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